Vive le difference: what drives intersexual differences in size and development time of Aedes mosquitoes?

Sexual size dimorphism (SSD) and sexual bimaturism (SBM) are widespread among animals, and can arise due to sexual selection, natural selection, or both. Dimorphism of size and age at maturity can be mediated by a difference in initial size, growth rate, development time, or asymptotic size of the sexes. Using Aedes mosquitoes as model organisms, two hypotheses for the evolution of SSD and SBM were tested: (1) direct selection on SBM, which posits that males and females gain direct fitness benefits from a difference in development time, and (2) indirect selection on SBM, where SSD is selected for and SBM merely a side effect due to constraint on growth curve components between sexes. By varying food availability and measuring disparity in adult emergence time and mass, this study tests the hypothesis that sexual bimaturism in A. aegypti, A. albopictus, and A. triseriatus is maintained by selection on different fitness components for males and females, and examines the proximate factors giving rise to the observed dimorphism in these mosquitoes. Our results provide more support for the indirect selection hypothesis due to the sensitivity of sexual bimaturism to food availability, indicating stronger fecundity selection on females to optimize mass gain during the larval stage than sexual selection on males to time emergence relative to females. Aedes albopictus and A. triseriatus mass increased proportionally with development time, but A. aegypti mass did not, indicating that an asymptotic size difference, a component of growth largely ignored in studies examining SSD, may be of primary importance in mediating the relationship between SSD and SBM in this species.